As a prior art of a cross-sectional shape of a gasket for a top cover in a hard disk unit, there is one described in patent document 1 (Japanese Patent Application Laid-open No. 2001-280507).
In this gasket, it is assumed to be preferable that, regarding the cross section in a direction generally perpendicular to the extension direction of a sealing material (gasket), Lm/Lp is 3 or less where the length of the contact surface with the cover member is Lp and the length of the non-bonded portion with the cover member is Lm.
However, in recent years, scale reduction of hard disk units is advancing and, in accordance therewith, it is inevitably becoming necessary to make a smaller gasket for a top cover.
Actually, as shown in FIG. 11, a gasket 101 is bonded to a gasket attachment surface having a small width.
When the gasket having a cross-sectional shape described in the patent document 1 is used in a top cover of such a hard disk unit having a reduced scale, a squeeze-out portion 102 that squeezes out to the inside of the hard disk unit is produced in a gasket 101, as shown in FIG. 12, at the time of attaching the top cover to a hard disk box body. This squeeze-out portion 102 of the gasket raises a problem of being brought into contact with a component in the inside, such as a disk.
On the other hand, when the whole is reduced in scale while maintaining the gasket having a cross-sectional shape described in the patent document 1 to have a ratio of Lm/Lp being 3 or less in order that the squeeze-out portion of the gasket does not raise a problem, the height of the gasket 201 is too small, as shown in FIG. 13, whereby the sealing property between the gasket 201 and the hard disk box body decrease, so that the gasket cannot prevent penetration of gas or dust, failing to exhibit a function as a gasket.
Also, the patent document 1 describes that “a good close adhesion property is obtained by letting the sealing material have a low hardness (70 degrees or less)”; however, when the sealing material is too soft, a suitable shape cannot be maintained and a buckling is produced at the time of compression depending on the cross-sectional shape of the gasket, thereby invoking poor sealing property and squeeze-out. Also, in order to reduce the hardness, a large amount of plasticizer must be used, and this causes generation of outgas. Adhesion of the components of the outgas to the disk causes an error operation of the hard disk. Further, too much softness causes poor moldability.